(a) Field of the Invention
This invention relates to an equalizer circuit capable of having a high D.C. (direct current) stability without worsening the equalization characteristic.
(b) Description of the Prior Art
Shown in FIG. 1 is a known typical circuit arrangement of equalizer circuits of the type in which an improvement of D.C. stability for a D.C. drift or the like is incorporated. Such a conventional equalizer circuit as shown in FIG. 1 comprises an equalizer amplifier 3 and a D.C. feedback circuit 6. The equalizer amplifier 3 comprises an amplifier 2 which is applied with a negative feedback through an equalizer element 1 (for example, an impedance element whose impedance value Ze corresponds to an inversed RIAA equalization characteristic). The D.C. feedback circuit 6 comprises an integration circuit 4 for integrating the output of the equalizer amplifier 3, and an inverter amplifier 5 for amplifying inversely in phase the output of the integration circuit 4. The D.C. feedback circuit 6 operates to feedback a portion of the D.C. components of the output of the equalizer amplifer 3 to the input thereof.
In the equalizer circuit constructed as above, an input signal applied to an input terminal 7 is amplified in accordance with a RIAA equalization characteristic to be delivered at an output terminal 8. In addition, the D.C. stability of the equalizer circuit is improved due to the provision of the D.C. feedback circuit 6.
Thus constructed equalizer circuit, however, has been found not satisactory in a practical use where the equalizer circuit has a high gain such as one used for a moving-coil type phono-cartridge. Since the low frequency cut-off characteristic of the D.C. feedback circuit 6 is characterized in a slow attenuation rate of 6 dB/oct, a sufficient attenuation degree is not obtained over an extreme low frequency range when the equalizer circuit having a high gain is used. As a result, the D.C. output of the equalizer circuit is susceptible to a D.C. drifting.
In view of the above problem, there has been proposed to set the lower cut-off frequency at a high frequency (e.g., over 1 Hz) in order to ensure a sufficient attenuation degree over the extreme low requency range. This proposed solution, however, results in a new problem that both low frequency characteristics of the D.C. feedback circuit 6 and the equalizer amplifier 3 interfere with each other thus to cause an undesirable deviation of the equalizer circuit characteristic from an ideal one throughout a low frequency range. A typical deviation characteristic of the equalizer circuit is illustratively shown in FIG. 2 wherein a solid line A designates such a deviation curve, f.sub.L and f.sub.M representing a first pole frequency and a second pole frequency, respectively corresponding to the frequencies 50 Hz and 500 Hz according to an RIAA standard.